The present invention relates to a vacuum fluorescent display using a surface electron-emitting source.
Conventionally, as a display component for an audio apparatus or automobile dashboard, a vacuum has been fluorescent display as one of electronic display devices frequently used. In the vacuum fluorescent display, an anode attached with a phosphor and a cathode at a position opposing the anode are arranged in a vacuum vessel, and light emission is obtained by bombarding electrons emitted from the cathode against the phosphor. Generally, a triode structure is used most often, in which a grid for controlling the electron flow is provided between the cathode and anode, so the phosphor selectively emits light.
In a conventional vacuum fluorescent display, a filament (filament cathode) obtained by applying an electron-emitting substance to a thin tungsten wire with a diameter of 7 μm to 20 μm is used as a cathode. The filament is attached to an elastic metal thin plate (anchor) fixed by welding to a pair of metal thin plates (filament supports) serving also as electrode leads. When a voltage is applied across the pair of filament supports so that a current is supplied to the filament, the heated filament emits thermoelectrons.
The emitted thermoelectrons are accelerated toward the anode and bombard against a phosphor film formed in a predetermined pattern, thus causing the phosphor to emit light. To turn on/off pattern display, the polarity of the voltage to be applied to the grid provided between the filament and anode is switched.
In the conventional vacuum fluorescent display, because the filament as described above is used as the cathode, the following problems arise.
Since a very thin, fragile filament must be attached in a taut state, it cannot be made long, and the display area cannot be increased. To uniform the luminance of the pattern to be displayed, the emitted electrons must be diffused by the grid. Therefore, it is difficult to obtain a high luminance.
In order to solve the above problems, a vacuum fluorescent display using a surface electron-emitting source as the cathode has been proposed. For example, a vacuum fluorescent display is known in which a surface electron-emitting source is formed as a cathode by printing a paste mixed with needle-like graphite columns with a length of several μm to several nm and made of an aggregate of carbon nanotubes. In a carbon nanotube, a single graphite layer is cylindrically closed, and a 5-membered ring is formed at the tip of the cylinder. Since the carbon nanotube has a typical diameter of as very small as 4 nm to 50 nm, upon application of an electric field of about 109 V/m, it can field-emit electrons from its tip. The surface electron-emitting source described above utilizes this nature.
FIGS. 7A and 7B show a conventional vacuum fluorescent display using a surface electron-emitting source as the cathode. As shown in FIG. 7A, the conventional vacuum fluorescent display has an envelope 400 constituted by a front glass member 401 which has light-transmission properties at least partly, a substrate 402 opposing the front glass member 401, and a frame-like spacer 403 for hermetically connecting the edges of the front glass member 401 and substrate 402. The interior of the envelope 400 is vacuum-evacuated. A light-emitting portion 410 with a predetermined display pattern is formed on the surface of the front glass member 401 in the envelope 400. The light-emitting portion 410 is constituted by a transparent electrode 411 arranged on the inner surface of the front glass member 401 to have a predetermined display pattern and serving as an anode, and a phosphor film 412 formed on the transparent electrode 411.
An electron-emitting portion 420 using carbon nanotubes as the electron-emitting source is formed on the surface of the substrate 402 in the envelope 400, at a position opposing the phosphor film 412, to have a pattern corresponding to the display pattern. An electron extracting electrode 430 with a large number of electron passing holes 431 is arranged between the electron-emitting portion 420 and phosphor film 412 to be spaced apart from the electron-emitting portion 420 by a predetermined distance. The electron extracting electrode 430 is supported by an insulating support member 440 provided on the edge of the electron-emitting portion 420. A front surface support member 405 vertically hanging toward the substrate 402 is formed on the surface of the front glass member 401 in the envelope 400 so as to surround the light-emitting portion 410. The front surface support member 405 is connected to an intermediate support member 406 formed on the edge of the electron extracting electrode 430.
In this arrangement, when a high voltage is applied across the electron-emitting portion 420 and electron extracting electrode 430 such that the electron extracting electrode 430 is set at a positive potential, the electric field is concentrated to the carbon nanotubes of the electron-emitting portion 420, and electrons are extracted from the tips of carbon nanotubes which are set at a high electric field. The extracted electrons are emitted through the electron passing holes 431 of the electron extracting electrode 430. For this reason, when a positive voltage (acceleration voltage) of, e.g., about +60 V is applied to the transparent electrode 411 with respect to the electron extracting electrode 430, electrons are accelerated toward the transparent electrode 411 and bombard against the phosphor film 412, thus causing it to emit light. Therefore, a predetermined display pattern is displayed.
In the conventional vacuum fluorescent display using a surface electron-emitting source, in order to increase the area of the display pattern, if the areas of the light-emitting portion 410 and electron-emitting portion 420 corresponding to the light-emitting portion 410 are increased, a phenomenon as shown in FIG. 8 occurs, in which only the peripheral portion of a display pattern 415 emits light brightly while light emission at the central portion of the display pattern 415 is dark. More specifically, a high-luminance portion 416 and low-luminance portion 417 are formed on the peripheral and central portions, respectively, of the display pattern 415, thus causing luminance nonuniformity in the display pattern 415.
In order to solve the above problems, the present inventors have studied factors that cause luminance nonuniformity in a large-area display pattern, and reached the following conclusion. According to the conclusion, as shown in FIG. 7B, when some of the electrons emitted from the electron-emitting portion 420 bombard against the insulating support member 440 between the electron-emitting portion 420 and electron extracting electrode 430, a larger number of secondary electrons than the electrons that have bombarded are emitted from the surface of the insulating support member 440, to charge the surface of the insulating support member 440 with a positive potential. When the insulating support member 440 is charged, the field strength in the vicinity of the insulating support member 440 increases, so electrons are easily emitted from the electron-emitting source in the vicinity of the insulating support member 440.
Therefore, the number of electrons bombarding against the peripheral portion of the phosphor film 412 close to the insulating support member 440 increases, and the peripheral portion of the phosphor film 412 emits light brightly. Accordingly, only the peripheral portion of the displayed pattern is bright while the central portion thereof is dark. The present inventors have made studies based on this conclusion, and found that the problems can be solved by actively utilizing charging of the insulating support member 440.